Contact type image sensor device with specific capacitance ratio

ABSTRACT

A contact type image sensor device comprises a plurality of photodiodes connected in a matrix, and an equal number of blocking diodes. A capacity ratio of a photo electric part constituting the photodiodes to a diode part constituting the blocking diode is in the range of 2:1 to 30:1. Dynamic range, magnitude of the signal current, light sensitivity and after image characteristics of the image sensor are improved according to the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a contact type image sensor device whichsenses an image of objects without reduction of the image. The contactimage sensor device is utilized for small-sized facsimile terminalequipment, a bar code reader, and the like.

2. Description of the Related Art

Since a contact image sensor device can sense an image of objectswithout use of an optical system for reduction of the image, the lengthof an optical path can be so shortened that equipment comprising thecontact image sensor device can be miniaturized. For this reason,recently, contact image sensor devices are widely used as an imagesensing unit in small sized facsimile terminal equipment, a bar codereader, and the like.

Though various types of contact image sensor devices are known, thecontact image sensor device of the present invention is a type of sensordevice which comprises a plurality of photodiodes connected in a matrixand an equal number of blocking diodes provided with each photodiode toprevent a so-called cross talk phenomenon. In this type of contact imagesensor device, both the photodiodes and the blocking diodes may beconstituted by diodes having the same construction, so that the degreeof integration of the sensor elements can be increased and they can beeasily fabricated.

U.S. Pat. No. 4,369,372 discloses a photo electro transducer devicewhich comprises n photo electro transducer elements and n diode elementsconnected respectively to the n photo electro transducer elements. Aphoto electro transducer element and a respective diode element areformed either on the same side of a common electrode area or formed inlayers.

Both the photodiode and the blocking diode have junction capacitancesC_(p) pl and C_(b), respectively. The capacitances C_(p) and C_(b) areequivalently connected to the photodiode and the blocking diode,respectively, in parallel In order to derive a signal current dependingon the brightness of a pixel from each photodiode connected in a matrix,a pulsed voltage is applied to the serially connected photodiode andblocking diode. Accordingly, the values of C_(p) and C_(b), especiallythe ratio of C_(p) to C_(b), is an important factor in determining adynamic range, magnitude of the signal current and light sensitivity ofthe image sensor device. However, these points are not described in theU.S. Pat. No. 4,369,372.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a contact image sensordevice comprising a plurality of photodiodes connected in a matrix, andwhich has improved dynamic range, light sensitivity and larger magnitudeof signal current.

In accordance with the present invention there is provided a contacttype of image sensor device comprising a plurality of photodiodesconnected in a matrix for converting a light signal to an electricalsignal depending on light intensity, and comprising an equal number ofblocking diodes to prevent a cross talk phenomenon, characterized inthat a capacity ratio of a photoelectric part constituting saidphotodiodes to a diode part constituting said blocking diodes is in therange of 2:1 to 30:1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical section view of a sensor element of a contact imagesensor device which is a first embodiment of the present invention;

FIG. 2 is a top view of the sensor element;

FIG. 3 and FIG. 4 are general cross-sectional views of sensor elementsof contact image sensor devices which are the second embodiments of thepresent invention;

FIG. 5 is a general plan view of a conventional two-dimensional sensorelement;

FIG. 6 is a general plan view of the sensor elements of the secondembodiment of the present invention;

FIG. 7 is a general cross-sectional view of a wiring of the sensorelements in the two-dimensional sensor, according to the presentinvention;

FIG. 8 is a plan view of the wiring;

FIG. 9 is a diagram showing an equivalent circuit of the contact imagesensor device;

FIG. 10 and FIG. 11 are timing charts for the equivalent circuit shownin FIG. 9; and

FIG. 12 is a diagram representing diode characteristics of the sensorelements, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first embodiment of the present invention is described referring toFIG. 1 and FIG. 2.

FIG. 1 is a vertical section view of a sensor element of a contact imagesensor device which is the first embodiment of the present invention,and FIG. 2 is a top view of the sensor element. In FIGS. 1 and 2,reference numeral 1 represents an insulating material, for example aglass substrate. A lower electrode 2 is arranged on the glasssubstrate 1. An amorphous silicon blocking diode (second diode) 5 havinga pin structure is formed on the left end of the lower electrode 2, andan amorhpous silicon photodiode (first diode) 6 having the samestructure is formed on the right end of the lower electrode 2. Upperelectrodes 3 are conjugated through a transparent conductive film, suchas ITO (Indium Tin Oxide) 7, on the blocking diode 5 and the photodiode6. Reference numeral 8 represents an insulating layer for insulatingbetween the sensor elements.

Since the blocking diode 5 and the photodiode 6 are formed on a commonlower electrode 2, a sensor element having a back-to-back connection iseasily fabricated so that the production process is simplified, in thefirst embodiment. Additionally, since the blocking diode 5 and thephotodiode 6 are separately formed, lateral movement of formed carriersdoes not occur, even if the blocking diode 5 and the photodiode 6 areconstructed as pin structure diodes made of an α-Si membrane.Furthermore, incident light from a light source (not shown) arranged onthe back side of the sensor element also occurs between the elements, sothat fabrication of a complete contact image sensor device is realized.

FIG. 3 to FIG. 8 show the second embodiment of the present invention.

FIG. 3 and FIG. 4 are general cross-sectional views of sensor elementsof contact image sensor devices which are the second embodiment of thepresent invention. In FIG. 3, reference numeral 4 denotes anintermediate electrode. The same reference numerals in the followingdescription as in the first embodiment are used for constituents havingsimilar operation and effect to those in the first embodiment, and thusexplanations thereof are left out. The sensor element shown in FIG. 3has a laminated structure of a blocking diode 5 and a photodiode 6,which is constructed by forming the blocking diode 5 and then forming,through a intermediate electrode 4, the photodiode 6 on the blockingdiode 5.

The sensor element shown in FIG. 4 has an integrated structure of ablocking diode 5 and a photodiode 6 on the blocking diode 5, which isconstructed by sequentially forming membranes of an n layer, i layer, player, i layer, and n layer on a lower electrode 2. In the sensorelement having the structure shown in FIG. 3 or FIG. 4, a capacitanceratio of the photodiode to the blocking diode can be altered by alteringa ratio of thickness of the i layers of diodes.

As the blocking diode 5 and the photodiode 6 are separately formed on anlower and upper side, the second embodiment has an operation and effectsimilar to the first embodiment.

FIG. 5 is a general plan view of a conventional two-dimensional sensorelement, and FIG. 6 is a general plan view of the sensor element of thesecond embodiment of the present invention. In the conventional sensorelement shown in FIG. 5, the blocking diodes 5 occupy a part of thesurface of the sensor element. However, only the photodiodes 6 occupythe surface of the sensor element in the second embodiment of thepresent invention as shown in FIG. 6, so that the ratio of an areaoccupied by a light receiving part to a unit pixel area, is increased.For example, in the case of a two-dimensional image sensor havingsixteen pixels/mm², when an area for a pixel forming region is 150 μmsquare, the light accepting area/unit pixel area is 25.8% in aconventional type sensor, and 35% in the present embodiment. The resultshows a 40% improvement in sensitivity.

FIG. 7 and FIG. 8 show a wiring of the sensor elements in thetwo-dimensional sensor. FIG. 7 is a general cross-sectional view andFIG. 8 is a plan view. In FIG. 7, reference numeral 9 denotes anon-glare membrane provided to improve light absorption. As shown inFIG. 7 and FIG. 8, according to the present invention, a floatingcapacitance between the wiring for output of a photo signal (upperelectrode 3), and the wiring for input of a switch signal (lowerelectrode 2) is more reduced than in a conventional sensor element,since the two wirings are separately arranged interposing thephotoelectric part and diode part.

Additionally, according to the present invention, sensitivity andresponsiveness are improved compared with a sensor element which isformed by arranging the photodiodes 6 and the blocking diodes 5 in aplane, since the wiring resistance between the photodiode 6 and theblocking diode 5 is extremely small.

A contact image sensor device of the present invention comprising theaforementioned sensor element is especially suitable for a deviceincluding a two-dimensional image sensor or a plurality of line sensors.

Next, a capacity ratio of the photodiode 6 to the blocking diode 5 inthe first and second embodiment of the present invention is described.FIG. 9 shows an equivalent circuit of the contact image sensor device.In FIG. 9, C_(p) is a junction capacitance of the photodiode 6, andC_(b) is a junction capacitance of the blocking diode 5. V_(j) is thepotential at B.

FIG. 10 and FIG. 11 are timing charts for the equivalent circuit shownin FIG. 9. FIG. 10 is the timing chart when C_(p) >>C_(b), and FIG. 11is the timing chart when C_(p) =C_(b).

When the photodiode 6 receives a light signal, current I_(p) flows andthe junction capacitance C_(p) is charged. Signal current I_(s) flowswhen a negative pulsed signal is applied to point A. The magnitude ofthe signal current I_(s) is measured as a time differential of ΔV_(j)(=V_(j1) -V_(j2)) which is the voltage drop at point B when the negativepulsed signal is applied to the point A. Accordingly, if ΔV_(j) islarge, the dynamic range of the signal current I_(s) becomes large andthe magnitude of the signal current I_(s) can be improved.

If the voltage of the pulsed signal applied to the point A is -5 V, thepotential at the point B just before application of the pulsed signal isV_(j0) ; the potential at the point B after a discrete potential changecaused by a capacitance C_(b) when the potential at the point A falls incorrespondence with a fall in the applied pulsed signal, is V_(j1) ; thepotential at the point B just before a rise of the pulsed signal isV_(j2) ; and the potential at the point B after a discrete potentialchange caused by a capacitance C_(b) in correspondence with a rise inthe pulsed signal, is V_(j3) ; the relationships among V_(j0), V_(j1),V_(j2), and V_(j3) are represented by the following equations.

    V.sub.j1 =[-5C.sub.b +(C.sub.p +C.sub.b)V.sub.j0  /(C.sub.p +C.sub.b)

    V.sub.j3 =[5C.sub.b +(C.sub.p +C.sub.b)V.sub.j2 ]/(C.sub.p +C.sub.b)

If C_(p) >>C_(b),

    V.sub.j1 =(-5C.sub.b /C.sub.p +V.sub.j0)>>(-5/2+V.sub.j0)

    V.sub.j3 =(5C.sub.b /C.sub.p +V.sub.j2)<<(5/2+V.sub.j2)

If C_(p) =C_(b)

    V.sub.j1 =(-5/2+V.sub.j0)

    V.sub.j3 =(5/2+V.sub.j2)

The capacity C of a capacitance is represented by the followingequation.

    C=εS/d

wherein S is a surface area of the capacitance, d is membrane thicknessof the capacitance, and εis a dielectric constant of the capacitance.

Accordingly, if a capacity ratio of the photodiode 6 to the blockingdiode 5 is large, the dynamic range of the signal current I_(s) is largeand the magnitude of the signal current I_(s) can be improved becausethe influence of the aforementioned discrete potential change is small.Namely, as the capacity ratio becomes large, the characteristics of theimage sensor is improved.

However, if the area ratio of the photodiode 6 to the blocking diode 5becomes too large, the area of the blocking diode 5 becomes relativelysmall, so that the forward current flowing in the blocking diode 5becomes small. To this end, since the discharge current of the junctioncapacitance C_(p) flowing through the blocking diode 5 in application ofthe pulsed signal becomes small, the drop of V_(j2) becomesinsufficient. Therefore, the value of ΔV_(j) and the signal currentI_(s) become small and an after image is increased.

The inventors fabricated an image sensor wherein the area of thephotodiode 6 is 105×125 μm² and the area of the blocking diode 5 is30×30 μm², namely, capacity ratio is 15:1. As a result of experiment,the signal current was I_(p) =5×10⁻⁶ A when illuminance was 1000 lux.When the capacity ratio was 1:1, signal current was I_(p) =1×10⁻⁶ A (1msec scan with -5 V, 24 μsec pulse). Additionally, as a result ofexperiments by the inventors, it was discovered that the after image isweak and the light sensitivity is excellent in the image sensor wherethe capacity ratio of the photodiode 6 to the blocking diode 5 is 2:1 isto 30:1, and therefore, the condition is suitable for the contact imagesensor device. In the second embodiment, however, in order to realizethe capacity ratio of 2:1 to 30:1, altering the membrane thickness ratioof i layers of the photodiode 6 to the blocking diode 5 as shown in FIG.4 is effective, as is altering the area ratio of the photodiode 6 to theblocking diode 5 as shown in FIG. 3. In the former case, it is possibleto keep the area of the blocking diode 5 sufficient, while keeping thecapacity ratio 2:1 to 30:1, so that the forward current flowing throughthe blocking diode 5 can be advantageously kept at a sufficient level.

Next, a membrane thickness of a photoelectric part and a diode part inthe first and second embodiments is described.

It has been previously known that if the membrane of the i layer is toothin, the photodiode 6 does not exhibit diode characteristics, and ifthe membrane of the i layer is too thick, a part of the i layer does notbecome a depletion layer. For this reason, conventionally, directcurrent bias voltage has been externally applied to the pin diode inorder to cause a sufficient number of light exited carriers to move,especially, in photodiode 6.

The inventors found from experiments that, when the membrane thicknessis 3,000Å to 10,000Å, the depletion layer extends over the whole i layerand the reverse current does not depend on the applied voltage, so thatapplication of the external direct current bias voltage is not required.As a result, a drive circuit for the image sensor device is simplified.Additionally, when the membrane thickness of the i layer is 3,000Å A to10,000Å, the ON voltage of the pin diode in a forward direction isrelatively low and switching characteristics of the pin diode areimproved in the blocking diode 5.

The inventors also found from experiments that, when the membranethickness of the p layer and the n layer is 100Å to 500Å, in thephotodiode 6, leakage current in the reverse direction can be suppressedto a sufficiently low level and a ratio of current from bright to darkis increased. The thickness of the p layer and the n layer which arelight insensitive layers, is so thin that sensitivity to short wavelengths is not remarkably decreased. Additionally, when the membranethickness is 100Å to 500Å, in blocking diode 5, leakage current in thereverse direction is small. As a result, the S/N ratio is improved and ahigh speed drive of sensor elements is realized.

Accordingly, it is preferable that the i layer of the photoelectric partand diode part be 3,000Å A to 10,000Å, and the p layer and n layer be100Å to 500Å.

The inventors measured the characteristics of a diode when the membranethickness of the p layer was 300Å, the membrane thickness of the i layerwas 6,000Å, and membrane thickness of the n layer was 300Å. The resultis shown in FIG. 12 (area of the diode=105×125 μm²). As shown in FIG.12, in the aforementioned diode, reverse light current does not dependon bias voltage, and reverse leakage current is less than 10⁻¹³ A.Additionally, the ON voltage in the forward direction is about 1 V. Anexperiment was performed regarding the contact type image sensor devicecomprising the photo-diodes 6 and the blocking diodes 5 constructed fromthe aforementioned diode. As a result, the S/N ratio is estimated to bemore than 35 dB.

We claim:
 1. A contact type image sensor device comprising a pluralityof photodiodes connected in a matrix for converting a light signal to anelectrical signal depending on light intensity and comprising an equalnumbers of blocking diodes to prevent a cross talk phenomenon, whereinacapacity ratio of a photoelectric part constituting said photodiodes toa diode part constituting said blocking diodes is in a range of 2:1 to30:1.
 2. A contact type of image sensor device as claimed in claim 1,wherein said photoelectric part and said diode part comprise a p layerhaving a membrane thickness of 100Å to 500Å, an i layer having amembrane thickness of 3,000Å to 10,000Å, and an n layer having amembrane thickness of 100Å to 500Å.
 3. A contact type of image sensordevice as claimed in claim 2, wherein both said photoelectric part andsaid diode part are separately formed on a common substrate.
 4. Acontact type of image sensor as claimed in claim 2, wherein saidphotoelectric part and said diode part are separately formed in alaminated structure.
 5. An image sensing device, comprising;a firstdiode for converting a light signal to an electrical signal depending ona light intensity, and a second diode for deriving the electrical signalfrom said first diode, wherein said second diode has a capacity ratio tosaid first diode in a range of 1/2 to 1/30.
 6. An image sensing deviceas claimed in claim 5, wherein said second diode has an area ratio tosaid first diode in a range of 1/2 to 1/30.
 7. An image sensing deviceas claimed in claim 5, wherein said first diode includes an amorphoussilicon device.
 8. An image sensing device as claimed in claim 5,wherein said second diode includes an amorphous silicon device.
 9. Animage sensing device as claimed in claim 7, wherein said first diodecomprises a p layer having a membrane thickness of 100Å to 500Å, an ilayer having a membrane thickness of 3,000Å to 10,000Å, and an n layerhaving a membrane thickness of 100Å to 500Å.
 10. An image sensing deviceas claimed in claim 8, wherein said second diode comprises a p layerhaving a membrane thickness of 100Å to 500Å, an i layer having amembrane thickness of 3,000Å to 10,000Å, and an n layer having amembrane thickness of 100Å to 500Å.
 11. An image sensing device asclaimed in claim 5, further comprising; common substrate for formingboth said first diode and said second diode separately on a surface ofsaid substrate.